When sand cores are used for making castings out of aluminum alloy or any other light alloy, the precision with which sand cores are positioned contributes in determining manner to the dimensional precision of castings made in a metal mold since said positioning is essential in determining the inside shapes of the casting, and also some of its outside shapes.
More precisely, and by way of example, when casting cylinder heads for motor vehicle engines, important engine functions are directly associated with the positioning of the cores: this applies in particular to the admission and exhaust ducts or pipes, which are made entirely by means of cores, and for which positioning precision has a direct influence on the performance of an engine (power, fuel consumption, polluting emissions, etc.).
Unfortunately, interaction between cores and metal mold elements raises problems which go against accurate control over dimensional positioning. Cores are generally made out of a mixture of sand (usually silica) of well-defined grain size and organic chemical binders which provide the core with cohesion and strength.
These binders are conventionally hardened in two broad families of core-making methods, either by using the “cold box” technique (i.e. using a gaseous chemical catalyst) or else the “hot box” technique (i.e. by delivering heat to the core box which is itself raised in temperature). However, regardless of which of those two core-making techniques is used, the cores behave in similar manner while casting is taking place. Thus, once they are placed in the mold which is itself already at a certain temperature, typically lying in the range 80° C. to 300° C. for its cooler portions and 400° C. to 500° C. for its hotter portions, the binders in the cores begin to decompose and to give off gaseous residues.
That process is then accelerated while liquid aluminum if being cast, since the aluminum penetrates into the mold at temperatures typically lying in the range 600° C. to 750° C.
The gaseous residues condense on the metal portions of the mold, locally building up successive layers of scale constituted by solid residues of said decomposition that are carbonized to varying extents.
Those residues are extremely hard and prevent cores being properly positioned on the metal portions.
To return to the example of admission or exhaust ducts in a cylinder head, the profile of the chamber is generally made by means of a cooled metal mold element serving to accelerate cooling of the aluminum locally during solidification, thereby locally refining its microstructure and improving its properties (strength, hot and cold fatigue performance, breaking elongation, etc.).
The cores that form the ducts stand on the cooled metal element. Thus, the scale accumulating on the contact surface of the metal mold element offsets the core ducts, thereby disturbing the precision with which they are positioned, and leading to the above-mentioned drawbacks.
In practical manufacture, that problem can be solved only by leaving substantial clearance for the guides and supports situated on the surfaces of the metal elements which interact with the facing surfaces of the cores, and by keeping these surfaces clean by cleaning them at regular intervals during manufacture, for example by brushing them.
Such operations disturb manufacture since they lengthen cycle times and damage the release coating that protects the mold elements from liquid aluminum, thereby requiring said coating to be repaired locally where necessary.
The manufacturer thus seeks to space such cleaning operations as far as apart as possible, but that goes against eliminating accumulations of scale.
Thus, in production, present practice is a compromise between those various constraints, thus putting a limit on the dimensional precision with which cores are positioned on metal elements.